This invention relates to a surface treatment apparatus and, more particularly, to an apparatus for inducing compressive residual stresses whereby the apparatus is capable of sensing variations in workpiece thickness and adjusting the applied surface treatment process parameters, such as the depth to which surface treatment elements are impinged, in response to such variations so that the desired depth and magnitude of compressive residual stress is induced in the surface of a workpiece. The invention also relates to a method of performing surface treatments whereby variations in workpiece dimensions are accounted for during the surface treatment process.
The high vibratory and tensile stresses experienced by rotating turbo machinery in operation, particularly the blading members of the fan, compressor, and turbine stages in gas turbine engines, make such components susceptible to high cycle fatigue (HCF) and other stress related failure mechanisms such as stress corrosion cracking (SCC). HCF and SCC ultimately limit the service life of these components as prolonged exposure to such extreme operating conditions leads to the development of fatigue cracks in areas of the component subject to high operational stresses. The fatigue life of a component is further limited by the occurrence of foreign object damage (FOD). FOD locations act as stress risers or stress concentrators that hasten the development and propagation of fatigue cracks. FOD, especially along the leading and trailing edges of blading members, significantly reduces the service life of aerospace components.
The impact of HCF, SCC, and FOD can be minimized and the service life of the component improved by using surface treatments to introduce compressive residual stresses in the surface of the component. Compressive residual stresses are commonly introduced in critical areas of the component that are subject to high operational stresses and/or damage and are therefore prone to fatigue failure. Such areas include the edges and tips of blading members. Introducing compressive residual stresses improves the fatigue properties and foreign object damage tolerance of both new and previously fielded blading members. The improvement in component properties through the introduction of compressive residual stresses decreases operation and maintenance costs and improves the flight readiness of the aircraft in which the component is employed. Common methods of introducing beneficial compressive residual stresses in aerospace components include laser shock peening (LSP), shot peening, pinch peening, glass bead peening, and burnishing.
Despite being well known in the art, such methods of inducing compressive residual stress can be difficult to implement, particularly with respect to vanes and blading members used in gas turbine engines. The difficulty lies not with the method of inducing the compressive residual stresses, but is instead related to the dimensional variations encountered within a single component as well as variations between individual components.
Current practices for the manufacture of fan and turbine blades and vanes for use in gas turbine engines allow for some dimensional variation. For example, in the case of some titanium alloy blades, variations in thickness on the order of several thousandths of an inch may be acceptable along the leading and trailing edges of the blade. Further, it is common practice to accept larger variations, in some cases nearly doubling the engineering tolerances, where the variations in thickness are in areas of the blade that are subject to lower stresses, such as near the tip of the blade. This practice is especially common where the manufacturer has invested significant resources in manufacturing the component such as with integrally bladed components and other complex components manufactured by electro-chemical machining.
Additional variations in the dimensions of the component may be introduced during the reworking or reconditioning of a blade or vane. FOD, such as nicks and dents along the edges of the blade, acts as a stress concentrator and may provide a location for crack initiation and growth that can ultimately lead to the failure of the component. Blades and vanes subject to FOD may be returned to service provided the damage can be removed through reworking or reconditioning the blade edge. In reworking the blade, the damaged area is removed by grinding or filing the edge of the blade around the damaged material and blending the material to facilitate a smooth transition between the original edge and the newly repaired area. The practice of grinding and blending effectively shifts the edge of the blade in the repaired area back from the original edge. Because the thickness of blades and vanes generally increases along the chord-wise direction towards the center of the airfoil and away from the leading and trailing edges, the edge of the repaired area is generally thicker than the surrounding areas.
Variations in the thickness of a blade or vane, regardless of the source, can adversely impact the benefit of compressive residual stresses induced in the blade. In general, a compressive residual stress distribution for a particular component is designed based on factors including the material from which the component is made, the applied and residual stresses to which the component is subject, and the dimensions of the component. The compressive residual stress distribution is designed such that a given magnitude of compression will extend to a specified depth beneath the surface of the part. For applications such as the edges of blades and vanes, it may be desirable to induce a compressive residual stress substantially through the entire thickness of the component over specified chord- and span-wise directions of the airfoil. Once the compressive residual stress distribution has been designed, process parameters are developed to induce the compressive residual stress distribution in the component.
Where variations in thickness are present, the surface treatment process parameters selected may be inadequate to produce the designed residual stress distribution. If the component is thicker than anticipated, the induced compressive residual stress may not reach the desired depth, have the desired magnitude, or extend through the thickness of the component where called for. Alternatively, if the component is thinner than anticipated, the amount of material in the treated area may not be sufficient to accommodate or compensate for reacting tensile stresses that accompany the introduction of compressive residual stress leading to buckling or distortion in the component.
One method currently employed to account for variations in thickness and dimension involves the use of a coordinate measuring machine (CMM) or similar apparatus before application of the surface treatment to precisely measure the dimensions of the component and identify any dimensional variations. Adjustments can then be made to the process parameters to account for any identified variations such that the processed component has the desired compressive residual stress distribution. While this method is effective, it has one significant disadvantage: the act of measuring prior to treatment adds an additional step to the surface treatment process that may as much as double the surface treatment process time.
Another problem associated with the introduction of compressive residual stress involves the ability to precisely control the applied stress or force of the surface treatment process so as to obtain the desired compressive residual stress. As shown in FIG. 4, for some materials, a small increase in the applied stress causes a substantial increase in the corresponding strain or deformation that develops in the material. This is schematically illustrated by the stress-strain curve 400 where a small increase in the applied stress (Δσ) 402 causes a substantial increase in the induced strain (Δε) 404. Because the induced strain, which is essentially the deformation of the material caused by the applied stress, is directly related to the residual compressive stress induced in the material, precise control over the applied stress may be necessary to achieve the desired residual compressive stress where small incremental changes in the applied stress cause comparatively large changes in the developed strain or displacement. However, it may be difficult to obtain the necessary precision over the applied stress to accurately control the induced compressive residual stress.
Therefore, a need exists for an efficient, easily incorporated surface treatment apparatus and method for identifying dimensional variations in components during a surface treatment process so that the surface treatment process parameters can be adjusted and the desired compressive residual stress distribution achieved with a high degree of precision.